A typical antenna set for portable wireless devices (such as, for instance, and without limitation, a handset, a mobile phone, a smartphone, a PDA, an MP3 player, a headset, a USB dongle, a laptop, a PCMCIA or a Cardbus 32 card), comprises at least one antenna element and a ground plane.
In general, said at least one antenna element includes a conductive plate or wire usually mounted on a carrier made of plastic (such as for instance Poly Carbonate, Liquid Crystal Polymer, Poly Oxide Methylene, PC-ABS, or PVC) that provides mechanical support, or instead directly mounted on the plastic enclosure of a portable wireless device (such as for instance a backcover). The antenna element is assembled in the portable wireless device, forming an integral part of the device. The portable wireless device will usually comprises a multilayer printed circuit board (PCB) which carries the electronics. One of the layers of the said multilayer PCB typically serves as a ground plane of the antenna set.
The trend in the sector of mobile phone manufacturers, and more generally portable wireless device manufacturers, is to integrate more and more mobile communication services (such as for instance, but not limited to, GSM850, GSM900, GSM1800, American GSM or PCS1900, GSM450, UMTS, WCDMA, or CDMA), together with added-value services allowing high data rate functionality (such as, for example, multimedia services, or on-line video), wireless connectivity and/or geolocalization (such as for example, but not limited to Bluetooth™, IEEE802.11a, IEEE802.11b, IEEE802.11g, WLAN, WiFi, UWB, ZigBee, GPS, Galileo, SDARs, XDARS, WiMAX, DAB, FM, DVB-H, or DMB) in more and more of their products At the same time, mobile phone manufacturers experience a strong market pressure to commercialize smaller-sized and less expensive devices.
These two market trends are in conflict because of the known trade-off between the performance of an antenna element and its size. As the size of handsets shrinks, and with it the space available to integrate the antenna element, the electrical performance of the antenna element (e.g., bandwidth, efficiency, or gain) degrades, thus hampering the ability of the antenna element to operate simultaneously in multiple bands. Furthermore, a ground plane of reduced dimensions (relative to the wavelength at the frequencies of operation of the antenna set) is of little help in enhancing the radiation properties of the antenna element.
Some attempts have been made to design antenna sets able to operate in multiple frequency bands, with an adequate electrical performance in each frequency band. They include the use of external monopole-type antennas, or antennas that comprise materials with high dielectric constant.
In the case of an external monopole antenna, the antenna element extends either partially or totally outside of the enclosure of the portable wireless device. Although multiband performance can be obtained, this solution is not satisfactory for small-sized portable wireless devices, because the external antenna implies an increase in the overall dimensions of these devices. Also, a wireless device using this type of antennas is likely to suffer from higher levels of specific absorption rate (SAR).
As mentioned, another possibility is the use of antenna elements comprising materials with high dielectric constant. Although these materials are effective in miniaturizing the dimensions of the antenna element, they are expensive and involve complex manufacturing processes, which make this approach unattractive for low-cost portable wireless devices.
Alternatively, rather than integrating all the operating bands in a single antenna element (which is likely to perform poorly in some bands), it is sometimes preferred to have two antennas inside the portable wireless device. For example, one antenna can be used to provide the GSM services, while another antenna can be used for UMTS. Although this approach alleviates the design complexity of each one of the antennas, in many cases it might not be practical, due to the little space available for the antennas inside the device, the increased complexity of the electronic circuitry to which the antennas are connected, and undesired coupling effects between the antennas. Moreover, such a solution can be expensive, and complicate the manufacturing process of a portable wireless device, since two antenna elements need to be provided and assembled into the device.
Another prior art solution is disclosed in WO-A-03/023900, further discussed below.